The present invention relates to a titanium alloy and a process for producing the same. Particularly, it relates to a titanium alloy, which can be utilized in a variety of products, and which exhibits a low Young""s modulus, a high elastic deformability and a high strength, and a process for producing the same.
Since titanium alloys are good in terms of the specific strength, they are used in the fields of aviation, military affairs, space and deep-see exploration, and so on. In the field of automobile as well, titanium alloys have been used in valve retainers, connecting rods, etc., of racing engines. Further, since the titanium alloys are good in terms of the corrosion resistance, they are often used under the corrosive environment. For example, they are used as materials for chemical plants, oceanic constructions, and the like, furthermore, for the purpose of inhibiting the corrosion, etc., due to the anti-freezing agents, they are used as automobile front bumper lowers, rear bumper lowers, and the like. Moreover, by focusing on the lightness (specific strength) and the anti-allergic property (corrosion resistance), the titanium alloys are used in accessories, such as wristwatches, etc. Thus, the titanium alloys are used in various and diverse fields, and, as representative titanium alloys, there are Ti-5Al-2.5Sn (xcex1 alloy), Ti-6Al-4V (xcex1xe2x88x92xcex2 alloy), Ti-13V-11Cr-3Al (xcex2 alloy), and so on.
By the way, the conventional titanium alloys have been often used while paying attention to the good specific strength and corrosion resistance, however, a titanium alloy (for example, the xcex2 alloy) has been often used recently while paying attention to the low Young""s modulus. For example, the titanium alloys of the low Young""s moduluses are used in organism compatible products (for instance, artificial bones, etc.), accessories (for example, frames of eyeglasses, etc.), sporting goods (for example, golf clubs, etc.), spring, and the like. When it is described by taking up concrete examples, in the case where the titanium alloy of the low Young""s modulus is used in an artificial bone, the Young""s modulus approaches a Young""s modulus of a human bone (to a degree of about 30 GPa), and the artificial bone becomes good in terms of the organism compatibility in addition to the specific strength and corrosion resistance. Further, a frame of eyeglasses, which comprises the titanium alloy of the low Young""s modulus, fits flexibly to a human body without giving an oppression feeling, and is good in terms of an impact absorbing property. Furthermore, when the titanium alloy of the low Young""s modulus is used in a shaft or head of a golf club, it is said that a flexible shaft and a head exhibiting a low intrinsic frequency can be obtained so that a driving distance of a golf ball extends. Moreover, when a spring, which comprises a titanium alloy exhibiting a low Young""s modulus, a high elastic deformability and a high strength, is obtained, a low spring constant can be achieved without increasing the number of turns, etc., and can be light-weighted and compacted.
Under these circumstances, the inventors of the present invention thought of developing a titanium alloy, which is intended to further expand the utilization in a variety of fields, and which exhibits a low Young""s modulus, a high elastic deformability and a high strength going beyond the conventional levels. And, first of all, they searched for the prior art concerning the titanium alloys, which exhibit the low Young""s modulus, and the following publications were discovered.
{circle around (1)} Japanese Unexamined Patent Publication (KOKAI) No. 10-219,375
In this publication, a titanium alloy, which contains Nb and Ta in a summed amount of 20-60% by weight. Concretely, to begin with, raw materials are melted so that the composition is achieved, and a button ingot is cast. Next, a cold rolling, a solid solution treatment and an aging treatment are carried out onto the button ingot. Thus, a titanium alloy, which exhibits a low Young""s modulus of 75 GPa or less, is obtained.
However, as can be understood from the examples disclosed in this publication, the tensile strength is lowered together with the low Young""s modulus, and a titanium alloy, which exhibits a low Young""s modulus, a high elastic deformability and a high strength, is not obtained. Moreover, on the cold working property, which is required to form the titanium alloy into products, there is no disclosure at all.
{circle around (2)} Japanese Unexamined Patent Publication (KOKAI) No. 2-163,334
In this publication, there is disclosed xe2x80x9ca titanium alloy comprising Nb: 10-40% by weight, V: 1-10% by weight, Al: 2-8% by weight, Fe, Cr and Mn: 1% by weight, respectively, Zr: 3% by weight or less, O: 0.05-0.3% by weight and the balance of Ti, and having a good cold working propertyxe2x80x9d.
Concretely, the titanium alloy having a good cold working property is obtained by carrying out a plasma melting, a vacuum arc melting, a hot forging and a solid solution treatment onto a raw material to be the composition.
However, on the Young""s modulus and the tensile strength, nothing is set forth in the publication. Moreover, by the titanium alloy, ln(h0/h): 1.35-1.45 is obtained as the maximum deformation ratio, at which no compression cracks occur, when this is converted into a cold working ratio later described, it is no more than about 50% at the highest.
{circle around (3)} Japanese Unexamined Patent Publication (KOKAI) No. 8-299,428
In this publication, a medical treatment appliance is disclosed which is formed of a titanium alloy comprising Nb of 20-40% by weight, Ta of 4.5-25% by weight, Zr of 2.5-13% by weight and the balance of Ti, and exhibiting a Young""s modulus of 65 GPa or less.
{circle around (4)} Japanese Unexamined Patent Publication (KOKAI) No. 6-73,475, Japanese Unexamined Patent Publication (KOKAI) No. 6-233,811 and Published Japanese Translation Publication (KOHYO) No. 10-501,719 of PCT International Publications for Patent Application.
In these publications, titanium alloys of low Young""s moduluses and high strengths are disclosed, however, concerning a titanium alloy exhibiting a Young""s modulus of 75 GPa or less and exhibiting a tensile strength of 700 MPa or more, there is disclosed a Ti-13Nb-13Zr only. In addition, on the elastic limit strength and the elastic deformability, nothing is disclosed at all. Moreover, in the scope of the claims, there is set forth Nb: 35-50% by weight, there is not disclosed at all a concrete example corresponding to it.
{circle around (5)} Japanese Unexamined Patent Publication (KOKAI) No. 61-157,652
In this publication, there is disclosed xe2x80x9ca metallic decorative article comprising Ti in an amount of 40-60% by weight and the balance of Nb substantiallyxe2x80x9d. Concretely, after arc melting a raw material having a composition of Ti-45Nb, it is subjected to a casting, a forging and rolling, and the resulting Nb alloy is subjected to a cold deep drawing, thereby obtaining a metallic decorative article. However, in the publication, nothing is set forth on a concrete cold working property at all.
Moreover, there are no descriptions on a Young""s modulus, a tensile strength, etc., of the Nb alloy.
{circle around (6)} Japanese Patent Publication (KOKAI) No. 6-240,390
In this publication, there is disclosed xe2x80x9ca material for a golf driver head comprising vanadium in an amount of from 10% by weight to less than 25% by weight, adjusting an oxygen content to 0.25% by weight or less, and the balance of titanium and inevitable impuritiesxe2x80x9d. However, a Young""s modulus of the used alloy is no more than about 80-90 GPa.
{circle around (7)} Japanese Patent Publication (KOKAI) No. 5-111,554
In this publication, there is disclosed xe2x80x9ca head of a golf club produced by a lost wax precision casting method with an Nixe2x80x94Ti alloy having a super elasticityxe2x80x9d. In this publication, there is set forth that Nb, V, etc., can be added a little, however, there is no description on their concrete compositions at all, moreover, there are not disclosed at all on a Young""s modulus, an elastic deformability and a tensile strength.
{circle around (8)} For reference, the Young""s moduluses of conventional titanium alloys are remarked additionally, the xcex1 alloy exhibits about 115 GPa, the xcex1+xcex2 alloy (for example, a Ti-6Al-4V alloy) exhibits about 110 GPa, and the xcex2 alloy (for example, Ti-15V-3Cr-3Al-3Sn), which is a4material subjected to a solid solution treatment, exhibits about 80 GPa, it exhibits about 110 GPa after it is subjected to an aging treatment. Moreover, when the inventors of the present invention examined and surveyed, the nickel-titanium alloy of the aforementioned publication {circle around (7)} exhibited the Young""s modulus of about 90 GPa.
The present invention has been done in view of these circumstances. Namely, as described above, the purpose is to provide a titanium alloy, which is intended to further expand the utilization in a variety of fields, and which exhibits a low Young""s modulus, a high elastic deformability and a high strength going beyond the conventional levels.
Further, the purpose is to provide a titanium alloy, which exhibits a low Young""s modulus and has a high elastic deformability as well as a high strength, and which exhibits a good cold working property so that it is readily formed into a variety of products.
Furthermore, the purpose is to provide a production process, which is suitable for producing such a titanium alloy.
The inventors of the present invention earnestly studied in order to solve this assignment, and carried out a variety of systematic experiments repeatedly, and, as a result, they completed to develop a titanium alloy, which comprises a predetermined amount of an element of Va group and titanium, and which exhibits a low Young""s modulus as well as a high elastic deformability and a high strength.
(1) Namely, a titanium alloy according to the present invention is characterized in that the titanium alloy comprises an element of Va group (the vanadium group) in an amount of 30-60% by weight and the balance of titanium substantially, exhibits an average Young""s modulus of 75 GPa or less, and exhibits a tensile elastic limit strength of 700 MPa or more.
By combining titanium and a proper amount of an element of Va group, a titanium alloy, which exhibits a low Young""s modulus unconventionally and has a high elastic deformability as well as a high strength. And, the present titanium alloy can be utilized widely in a variety of products, and it is possible to intend the improvements of their functional properties and the enlargements of their designing freedom.
Here, the element of Va group is adjusted to 30-60% by weight, because a sufficient decrement of an average Young""s modulus is not intended when it is less than 30% by weight, on the other hand, when it exceeds 60% by weight, a satisfactory elastic deformability and tensile strength are not obtained, and the density of the titanium alloy rises to decrease the specific strength. Moreover, when it exceeds 60% by weight, it is likely to cause not only the decrement of the strength but also the decrements of the toughness and ductility, because the material segregation is likely to take place to impair the homogeneity of the material.
And, the inventors of the present invention confirmed that this titanium alloy is provided with a good cold working property.
It is not clear still why the titanium alloy of that composition exhibits a low Young""s modulus and a high elastic deformability as well as a high strength, and why it is good in terms of a cold working property. According to the surveys and researches, which were carried out so far by the inventors of the present invention, on their properties, it is possible to think as follows.
Namely, as a result of a survey, which was carried out by the inventors of the present invention, on a sample according to the titanium alloy of the present invention, it was proved that, even when this titanium alloy was subjected to a cold working process, the dislocation is hardly introduced, and the titanium alloy showed a structure whose (100) plane was oriented very heavily in a part of direction. Besides, in the dark field image employing the 111 diffraction point, which was observed by a TEM (Transmission Electron Microscope), it was observed that the contrast of the image moved together with the inclination of the sample. This indicates that the observed (111) plane is curved, and this was also confirmed by a lattice image direct observation of a high magnification. In addition, the curvature radius of this curve of the (111) plane was extremely small, and was 500-600 nm approximately. This means that the titanium alloy of the present invention relieves the influences of workings, not by the introduction of the dislocation, but by the curve of the crystal plane, and that it has a quality, which has not been known at all in conventional metallic materials.
Further, the dislocation was observed in a very extreme part, while the 111 diffraction point was heavily excited, but was hardly observed when the excitation of the 111 diffraction point disappeared. This indicates that the displacement components around the dislocation are biased remarkably in the  less than 110 greater than  direction, and this suggests that the titanium alloy of the present invention exhibits a very heavy elastic anisotropy. The reason is not clear, but it is considered that this elastic anisotropy closely relates to the good cold working property, the appearance of the low Young""s modulus, the high elastic deformability and the high strength, and the like, of the titanium alloy according to the present invention.
Note that the group Va element can be one kind or a plurality of kinds of vanadium, niobium and tantalum. All of these elements are xcex2-phase stabilizing elements, however, it does not necessarily mean that the titanium alloy of the present invention is conventional xcex2 alloys.
Furthermore, heat treatments are not required necessarily, but it is possible to intend to further highly strengthen by heat treatments.
Moreover, the average Young""s modulus can be preferable so that it is 70 GPa or less, 65 GPa or less, 60 GPa or less and 55 GPa or less in this order. The tensile elastic limit strength can be preferable so that it is 750 MPa or more, 800 MPa or more, 850 MPa or more and 900 MPa or more in this order.
Here, the xe2x80x9ctensile elastic limit strengthxe2x80x9d is referred to a stress, at which a permanent strain reaches 0.2%, in a tensile test, in which a load is applied to and removed from a test piece gradually and repeatedly. It will be described later in more detail.
In addition, the xe2x80x9caverage Young""s modulusxe2x80x9d does not refer to the xe2x80x9caveragexe2x80x9d of Young""s modulus in the strict sense, but it means a Young""s modulus, which represents the titanium alloy of the present invention. Concretely, in a stress (load)-strain (elongation) diagram, which is obtained by the aforementioned tensile test, a gradient (gradient of tangent line) of a curve at a stress position, which corresponds to xc2xd of the tensile elastic limit strength, is referred to as the average Young""s modulus.
By the way, the xe2x80x9ctensile strengthxe2x80x9d is a stress, which is obtained by dividing a load immediately before a final breakage of the test piece by a cross-sectional area of the parallel portion of the test piece before the test.
Note that the xe2x80x9chigh elastic deformabilityxe2x80x9d in the present application means that the elongation of the test piece is large within the aforementioned tensile elastic limit strength. Further, the xe2x80x9clow Young""s modulusxe2x80x9d in the present application means that the aforementioned average Young""s modulus is smaller with respect to the conventional and general Young""s modulus. Furthermore, the xe2x80x9chigh strengthxe2x80x9d in this application means that the aforementioned tensile elastic limit strength or the aforementioned tensile strength is large.
Note that the xe2x80x9ctitanium alloyxe2x80x9d in the present invention includes a variety of forms, and that it means not only workpieces (for example, ingots, slabs, billets, sintered bodies, rolled products, forged products, wire rods, plates, rods, etc.) but also the titanium alloy members (for example, intermediately processed products, final products, parts of them, etc.), in which they are processed (hereinafter, the meanings are the same.).
(2) Alternatively, the titanium alloy of the present invention is characterized in that the titanium alloy is a sintered alloy comprising an element of Va group (the vanadium group) in an amount of 30-60% by weight and the balance of titanium substantially.
The present invention is based on a discovery that sintered alloys (sintered titanium alloys), which comprised titanium and proper amounts of the group Va elements, had such mechanical properties that they were of low Young""s modulus and exhibited high elasticity deformabilities and high strengths.
And, the inventors of the present invention confirmed that this titanium alloy was provided with a good cold working property. The reason why the Va group element is adjusted to 30-60% by weight is as aforementioned.
It is not still clear why the titanium alloy of the composition exhibits low Young""s modulus, high elasticity deformability and high strength, and why it is good in terms of the cold working property, however, at present, the reasons are believed as aforementioned.
(3) A process for producing a titanium alloy according to the present invention is characterized in that the process comprises the steps of: a mixing step of mixing at least two or more raw material powders containing titanium and an element of group Va in an amount of 30-60% by weight; a compacting step of compacting a mixture powder obtained by the mixing step to a green compact of a predetermined shape; and a sintering step of sintering the green compact obtained in the compacting step by heating.
The production process of the present invention (hereinafter, it is referred to as a xe2x80x9csintering processxe2x80x9d wherever appropriate.) is suitable for producing the aforementioned titanium alloy.
As can be understood from the aforementioned patent publications, etc., the conventional titanium alloys are often produced by casting after melting a titanium raw material (for example, a sponge titanium) and an alloy raw material, and thereafter by rolling the resulting ingots (hereinafter, this process is referred to as a xe2x80x9cmelting processxe2x80x9d wherever appropriate.).
However, since the titanium has a high melting point and is very active at elevated temperatures, it is difficult to carry out the melting itself, and there often arise cases where special apparatus are required to carry out the melting. Further, it is difficult to control the compositions during the melting, and it is necessary to carry out the multiple melting, and so on. Furthermore, a titanium alloy, such as the titanium alloy of the present invention, containing large amounts of the alloy components (particularly, the xcex2-stabilizing elements), is less likely to avoid the macro segregations of the components, and a stable quality titanium alloy is difficult to obtain.
On the other hand, in accordance with the sintering process of the present invention, since it is not necessary to melt the raw materials, there are no disadvantages like the melting process, and it is possible to efficiently produce the titanium alloy according to the present invention.
To put it concretely, since the raw material powders are mixed uniformly by the mixing step, a homogeneous titanium alloy can be readily obtained. Further, since a green compact having a desired shape from the beginning can be compacted by the compacting step, the production steps can be significantly reduced. Note that the green compact can be compacted as workpiece shapes, such as plates, rods, etc., can be compacted as shapes of final products, or shapes of intermediate products before reaching them. And, in the sintering step, the green compact can be sintered at temperatures considerably lower than the melting points of titanium alloys, no special apparatuses like those of the melting process are required, and, moreover, it is possible to carry out an economical and efficient production.
Note that the production process of the present invention uses two or more raw material powders in view of the mixing step, and is based on the so-called blended elemental (mixing) method.
(4) A process for producing a titanium alloy according to the present invention is characterized in that the process comprises the steps of: a packing step of packing a raw material powder containing titanium and at least an element of group Va in an amount of 30-60% by weight into a container of a predetermined shape; and a sintering step of sintering the raw material powder in the container by using a hot isostatic pressing method (HIP method) after the packing step.
In accordance with the production process of the present invention, the aforementioned mixing step and/or the compacting step are not required necessarily. Moreover, in accordance with the production process of the present invention, the so-called pre-alloyed powder metallurgy method can be carried out. Accordingly, the kinds of usable raw material powders are broadened, not only mixture powders, in which two or more of pure metallic powders and/or pre-alloyed powders are mixed, but also pre-alloyed powders, which have the aforementioned or later described compositions of the titanium alloys of the present invention, can be used. And, by using the HIP method, dense sintered titanium alloys can be obtained, and, even if the product shape is complicated, the net shape can be carried out.
Note that the composition ranges of the aforementioned respective elements are shown in a form of xe2x80x9cx-y % by weightxe2x80x9d, unless otherwise specified, it means to include the lower limit value (x % by weight) and the upper limit value (y % by weight).